When Dr. Edward Angle first introduced the brackets and archwires for the edgewise mechanism in 1925, the archwires were made of gold and the brackets from a solid block of metal. Dr. Angle originally claimed his edgewise archwire was “ . . . exactly 0.022 inch in thickness and 0.028 inch in width, and it most accurately fits the slots in brackets.” (Angle, E. H., The latest and best in orthodontic mechanism. Dental Cosmos 1928;70:1143–1158.) Dr. Angle's use of the term “accurately” leaves the reader with the assumption that the slot is also 0.022 inch. Of course, if this were the case, it would have been nearly impossible to insert the wires into the slots.
Accordingly, even though the same sizes are listed for slots and wires, the slots must be a little larger and/or the wires slightly smaller. It has become accepted procedure to manufacture archwire slots oversize vertically from 0.001 inch to 0.002 inch and the rectangular archwires 0.0005 inch undersize. Therefore, all edgewise archwires are undersize vertically as compared to their respective archwire slots.
The differences between the various listed and advertised sizes, and the actual rectangular wire and slot sizes, and the resulting vertical tolerances are illustrated in the following chart:
SlotWire SizeWire SizeSizeSlot SizeVertical(Listed)(Actual)(Listed)(Actual)Tolerance0.018″ × 0.025″0.0178″ × 0.0251″0.018″0.0189″0.0011″0.019″ × 0.025″0.0188″ × 0.0247″0.019″00(none)0.0215″ × 0.028″0.0214″ × 0.0279″0.022″0.0231″0.0017″
This amount of tolerance or play is necessary to facilitate the placement and removal of archwires. The resultant lack of exact axial control with the edgewise mechanism has been accepted as a “fact of life” in orthodontics for the past eighty years.
The following definitions of dental directional terms will be referred to clarify the understanding of the invention:
Mesial—toward the front of the dental arch
Buccal/labial—toward the cheek or lip
Palatal/lingual—toward the palate or tongue
Tip—inclination of bracket or tooth in mesial-distal direction
Torque—inclination of bracket or tooth in labial-lingual direction
Occlusal/incisal—toward the biting surface of tooth
Gingival—toward the gums.
However, this play can be eliminated by the orthodontist when placing small bends in the archwire to achieve desired final degrees of tip or torque. By placing vertical steps (second order bends) in the wire on either side of the bracket to tip the tooth mesially or distally, the wire is angled in relation to the level slot and makes contact with diagonally opposed ends of the slot. Of course, the wire must actually be overbent to overcome the vertical play and deliver the desired degree of tooth tip. This, of course, requires much skill and thought on the part of the orthodontist.
To achieve the desired degree of torque the orthodontist must twist the rectangular archwire about its long axis (third order adjustment). These bends are time-consuming and the flexing required to place the bent wires into the slots can be very uncomfortable for the patient. Such wires often require the use of special seating tools or torquing keys to insert them into the slots.
This was the recommended practice in orthodontics when the edgewise appliance was originally introduced by Dr. Angle in the 1920's and continued by Dr. Charles Tweed and others through the 1960's.
In the early 1970's the idea of preadjusted archwire slots became popular, as advocated by Dr. Lawrence Andrews (Andrews, L. F., The six keys to normal occlusion. Am J Orthod 1972;62:296.) The concept is to angle the archwire slots both mesiodistally (for desired tip) and buccolingually (for desired torque) so that “straight” archwires can be used with no need for bending by the orthodontist. The popular term for this concept is the straight-wire appliance because theoretically there are no individual bends required for each tooth. Prescriptions of differing degrees are provided to treat various types of patients.
When referring to a “prescription” herein, it will be understood to mean a system to be followed by an orthodontist when treating a patient to obtain a desired end result. Such a system would use a series of brackets at least some of which would have built in degrees of tip and torque for each of the teeth on which those brackets are to be mounted.
However, this created another problem. In the absence of the overbending described above, the vertical play or tolerance between the wire and the slot and its associated lack of torque and tip control became more significant. As shown below, the play (degrees of lost control) is much greater in regard to torque than tip.
When using straight “full-size” edgewise archwires in their respective slots, the following torque and tip play is produced:
Wire SizeSlot SizeTorqueTip-Play(Listed)(Listed)Play0.075″ Wide0.160″ Wide0.018″ × 0.025″0.018″2.6 Degrees1.2 Degrees0.4 Degrees0.0215″ × 0.028″0.022″4.0 Degrees1.8 Degrees0.6 Degrees
Actually, the range of torque and tip play is double the degrees listed above as the tooth/bracket can rotate back and forth about the archwire in two directions.
Accordingly, the necessary slot and archwire size tolerances required to facilitate placement of archwires in the bracket slots results in lost tip and torque control. To compensate for lost torque control due to vertical tolerances between the archwire and a conventional edgewise slot, the straight wire torque prescription (angulation) can be “over done.” In other words, when −12 degrees of palatal root torque is desired for the maxillary central incisors and there is 4 degrees of play when using a “full-size” 0.0215″×0.028″ archwire in a 0.022″ slot, the torque prescription in the base of the bracket or the slot is increased to −16 degrees. Theoretically then, when using a flat, full-size archwire, the maxillary central incisors will be moved to, or held in axial positions of −12 degrees of palatal root torque.
In 1986 Peter C. Kesling invented the Tip-Edge® bracket as disclosed in U.S. Pat. No. 4,877,398, and developed the Tip-Edge® technique for orthodontically treating a patient. This Tip-Edge® bracket includes a unique archwire slot that initially permits up to 30 degrees of tipping and finally provides 100 percent of both torque and tip control. Tip-Edge® is a registered trademark to TP Orthodontics, Inc., of Westville, Ind.
Unlike a conventional edgewise slot with directly opposed parallel walls, there is no control of tip or torque without an uprighting spring or the threading of an auxiliary wire through a tunnel, as disclosed in U.S. Pat. No. 6,682,345. Because the unique Tip-Edge slot does not bind on the archwire when sliding teeth/brackets mesially or distally, there is no need to use undersize archwires in the slot to facilitate tooth movement.
Dr. Kesling also disclosed in 1997 (Kesling PC. Vertical slots—expanding the versatility of edgewise brackets. Video April 1997) that through the power of a mesiodistal uprighting spring, a bracket with a conventional edgewise slot could be rotated in conjunction with a full size rectangular archwire to provide 100 percent torque control. However, because of a general dislike for uprighting springs that are difficult to use, unsightly, uncomfortable and unhygienic, few orthodontists have attempted to capture lost torque in this manner. Such a procedure has evidently never been suggested in the literature.
In recent years this problem of lost torque due to the vertical tolerances between the wire and a conventional edgewise archwire slot has been exacerbated by the practice of using substantially undersize rectangular archwires in the straight-wire technique. Archwires of 0.019″×0.025″ are employed in 0.022″ slots. The increase in tolerance is necessary to facilitate bodily sliding the brackets/teeth along the wire. It also aids archwire engagement and reduces patient discomfort.
However, no such use of substantially undersize archwires has proved necessary when using Tip-Edge brackets because as mentioned before the Tip-Edge slot does not bind on the archwire. It, in effect, increases in size vertically as brackets/teeth are slid along the full-size wire.
The use of 0.019×0.025 archwires in 0.022 slots results in the following ranges of torque and tip play:
ActualRange ofListedListedVerticalRange ofTip-PlayWire SizeSlot SizeActualActualToleranceTorque Play0.160″ Wide Brackets0.075″ Wide Brackets0.019″ × 0.025″0.022″0.0188″ × 0.0247″0.0231″0.043″21.6 Degrees3.2 Degrees9 Degrees
The increase in lack of tip and torque control caused by the ranges of play above has been clearly recognized in the leading straight-wire textbook and reported in the literature (McLaughlin, R. P., Bennett, J. C., Trevisi, H. J. Systemized orthodontic treatment mechanics. Edinburgh, Mosby, 2001. Kapur-Wadhwa, R. Physical and mechanical properties affecting torque control. J Clin Orthod 2004;38:335–340.)